What's the fast way to multiply by 5, 25, 50, and 125?

Each is a power of ten divided by a small number, so multiply by the power of ten and divide.

Since 5 = 10⁄2, 25 = 100⁄4, 50 = 100⁄2, 125 = 1000⁄8:

• ×5: ×10 then halve. 48 × 5 = 480⁄2 = 240.
• ×25: ×100 then ÷4. 48 × 25 = 4800⁄4 = 1200.
• ×50: ×100 then ÷2. 48 × 50 = 2400.
• ×125: ×1000 then ÷8. 48 × 125 = 48000⁄8 = 6000.

Do the divide-then-scale order if the number halves cleanly (48⁄2 = 24, ×10 = 240).